1. Technical Field
The present invention relates generally to a signal monitoring apparatus for wavelength division multiplexed (hereinafter, referring to WDM) optical telecommunication networks, particularly to an optical channel error monitoring and detecting apparatus capable of effectively operating optical telecommunication networks by maintaining a wavelength, which is allocated to each certain WDM channel, and an optical power in a constant error range as well as extending number of WDM channels. It is performed by using a reference signal having a wide wavelength tunable range and a repeatability to a wavelength such that the monitoring system may monitor and revise the wavelength of each WDM channel and the optical output error.
2. Background
Recently, there has been considerable interest to increase the optical telecommunication capacity by using an optical fiber. Thereby, the WDM is introduced among methods capable of handling multichannel optical signals in an optical fiber. And also there is a great amount of study towards minimizing and revising the problems which affect the adjacent channels according to the errors of the channel wavelength and the optical power.
There are a lot of studies to control each WDM channel wavelength. Recently, research into the prevention of crosstalk by maintaining all channels to have the same optical power has begun by measuring each WDM channel optical output.
FIG. 1 shows a schematic diagram of a general WDM optical transmission terminal which comprises a WDM transmitter module 300, an optical channel error monitoring and detecting device 100, and an optical transmitter controller 400. A WDM optical transmitter 310 is a transmitting module which is comprised of a plurality of optical transmitters 320 having optical wavelengths which are different from each other. Here, each optical transmitter 320 is a laser generator for converting an electrical signal to an optical signal of a specified optical wavelength. And an optical combiner 330 is a directional coupler for transmitting the wavelength multiplexed optical signal to a single optical fiber after the wavelength multiplexed optical output of each optical transmitter 320. An optical divider 340 divides the optical output of the optical combiner 330 by a constant rate. One of the divided optical outputs is used for a WDM optical telecommunication. The other optical output is connected to the optical channel error monitoring and detecting device 100 and used for an optical channel error monitoring and detecting. The optical channel error monitoring and detecting device 100 transmits monitoring information of the optical channel to the optical transmitter controller 400. The optical transmitter controller 400 controls each optical transmitter 320 and revises the error by using the monitoring information.
FIG. 2 shows a schematic diagram explaining conventional optical channel error monitoring and detecting of a WDM system. And FIG. 3 shows an illustration for understanding an optical channel wavelength detection according to the prior art, in which FIG. 3 relates to the optical channel wavelength detection by using a wavelength tunable laser (hereinafter, referring to WTL) 110 and an optical resonator (filter).
Now the prior art will be explained with reference to FIG. 2, and FIG. 3.
The wavelength tunable laser (WTL) 110 is a laser generator for converting an electric signal to an optical signal of a certain optical wavelength. And the optical wavelength is controlled by an electric signal. An optical divider 120 divides the optical signal generated from the WTL 110 and outputs the divided optical signal with a same rate (1:1) to a pair of outputs. An optical combiner 125 couples the optical signal of each WDM channel and the divided signal from the WTL 110. The optical fiber 130 shows a transmitting state of the multiplexed WDM optical signal. And an etalon resonator 140 is an optical filter of the resonator type. The etalon resonator 140 passes the optical signal of the WTL 110 whenever the optical wavelength of the WTL 110 matches a resonant frequency.
A photodiode 151 detects a beating signal between the optical signal of the WTL 110 and each WDM channel. And the other optical receiver 152 detects an optical signal of the WTL 110, when the optical wavelength of the WTL 110 matches to the resonant frequency. A timing comparator 160 detects and compares an arrival time between the detected optical signals from each optical receivers 151, 152. And it is ideal that signal detecting time of two optical receivers 151, 152 are same.
Now it will be explained about the operation of the prior art which uses the WTL 110 and the etalon resonator 140 for controlling the WDM channel wavelength described above, at the same time.
The resonant frequency of the etalon resonator 140 is a standard frequency of each WDM channel signal, in which the optical transmitter for each WDM channel should be transmitted during a stable state.
The optical signal generated from the WTL 110, is separated into a pair of signals having the same power in the optical divider 120. One signal of the pair of signals is coupled with each WDM channel signal in the optical fiber 130. And the other signal is passed through the etalon resonator 140 and generates a standard frequency of the WDM channel, in which each WDM channel optical transmitter is operated in the stable state.
Here, an optical signal of WTL 110 which is passed through the optical divider 120 and inputted to the optical fiber 130, is coupled with the optical signal of each WDM channel. Accordingly, an optical receiver 151 detects a beating signal when the optical wavelength of each WDM channel signal and the optical wavelength of the WTL 110 are same. And the optical receiver 151 measures frequencies of each WDM channel by using the detected beating signal.
Additionally, the optical signal of the WTL 110 which has passed through the optical divider 120 and applied to the etalon resonator 140, detects the standard frequency in the optical receiver 152, by passing the optical signal only whose wavelength is the same as resonant wavelength of the etalon resonator 140 (the resonant wavelength of the etalon resonator 140 is the standard frequency of each WDM channel). Here, the resonant wavelength of etalon resonator 140 is synchronized with the standard frequency of each WDM channel, precisely.
The detecting time from the optical receiver 151 is compared with the detecting time from the optical receiver 152. And thereby, it is possible to detect an error of the optical wavelength of each WDM channel signal.
The WTL 110 sweeps the wavelength region of WDM channel at a constant cycle, and thereby detects the error.
It is ideal that a signal detecting time of the optical channel detected from the respective optical receivers 151, 152, is the same compared to the timings comparator 160.
For example, a detecting time of the certain optical channel among the WDM channels detected from the optical receiver 151 exists slower or faster than the detecting time of the corresponding reference optical signal of the optical receiver 152, when an optical wavelength of the optical channel has longer or shorter wavelength than the standard frequency which is resonated from the etalon resonator 140.
Accordingly, it is possible to revise/complement an error signal of the time difference measured from the timing comparator 160, by controlling an optical transmitter driving unit for each WDM channel.
The conventional error monitoring and detecting technology of the optical telecommunication system, using the WTL 110, has problems which limit channel number of the WDM optical telecommunication system according to a wavelength tunable range of the WTL 110.
For example, in case that the channel spacing is 200 GHz, and the channel number is 16, a current wavelength tunable range is considerably limited even though the wavelength tunable laser (WTL) 110 should vary the optical wavelength of 3200 GHz.
Additionally, there is no way to confirm a stability because a mode hopping may be generated in the WTL 110 which is not used commonly. So it affects an efficiency of the system in case of monitoring and controlling the optical wavelength of the WDM optical telecommunication system.
Accordingly, in order to solve the problems in the prior art, the present invention provides a signal monitoring apparatus for wavelength division multiplexed (WDM) optical telecommunication networks capable of efficiently maintaining an operating wavelength, in which each WDM channels are allocated in a certain way, and an optical power in a constant error range without limiting the number of the WDM channel. It may monitor and revise an error of an operating wavelength of each WDM channel and of the optical output by using a reference signal which has a wide wavelength tunable range and a repeatability to a wavelength.
One embodiment to achieve the object in accordance with the present invention is to provide a signal monitoring apparatus for wavelength division multiplexed (WDM) optical telecommunication networks, comprising absolute wavelength reference generating means for generating an absolute wavelength reference stabilized on an absolute wavelength reference, wavelength reference generating means for generating a wavelength reference set having a constant frequency spacing from a wide band optical signal, tunable filtering means for receiving the absolute wavelength reference or one wavelength reference among the wavelength reference set of the constant frequency spacing, or a wavelength division multiplexed (WDM) channel signal which is used in the optical telecommunication networks, and control means for counting a gap between the absolute wavelength reference and the WDM channel signal a certain frequency value, measuring the optical power of the WDM channel and controlling the wavelength and the optical power of the WDM channel, in which the absolute wavelength reference passes the tunable filter means.
Another embodiment to achieve the object in accordance with the present invention is to provide a signal monitoring apparatus for WDM optical telecommunication networks. Wherein the wavelength reference generating means further comprises an isolator for preventing an external optical signal from inputting to the wide band signal generating means.
The other embodiment to achieve the object in accordance with the present invention is to provide a signal monitoring apparatus for WDM optical telecommunication networks, further comprising at least one optical receiving means for detecting optical signals which pass the tunable filtering means and converting the detected optical signals to an electric signal, wavelength reference generating means of generating a wavelength reference set having the constant frequency spacing, and the tunable filtering means respectively, and at least one optical dividing coupler means for connecting the absolute wavelength reference generating means.